A reluctance motor, unlike an induction motor, produces no secondary copper-loss of its rotor. Because of this feature, the reluctance motor has been contemplated as a driving motor for electric vehicles and machine tools. However, the reluctance motor generally outputs low power, so that its rotor-core structure or driving method needs improvement before it can be put into practical use. Recently, a technique for increasing the power output has been developed. According to this technique, a multi-layered flux barrier is prepared on a core sheet of a rotor core, as disclosed in 1996 National Convention Record I.E.E. Japan, Item 1029, Development of Multi-flux Barrier Type Synchronous Reluctance Motor by Yukio Honda, et al.
FIGS. 12A-12C show a structure of a rotor-core of this conventional, but improved reluctance motor. In FIG. 12A, a disc-shaped core sheet 161 made from an electromagnetic steel sheet has a multi-slitted flux-barrier 162 having an inverse-arc configuration, with respect to an axis 163 of the core sheet 161. The flux-barrier 162 is made up of slits (through-slots) having approximately 1 mm width each (which can be formed by press working). Each slit has an end 164 that terminates within a given width inside its outer rim to strengthen the outer rim against centrifugal force applied thereto when the core sheet 161 spins.
Several tens of core sheets 161 are laminated along a rotor shaft 165, thereby completing a rotor core 166 as shown in FIG. 12B. Setting this rotor-core 166 in a stator 167 as shown in FIG. 12C causes a plurality of field-magnet sections 168 to impart a rotating magnetic field to the rotor core 166, thereby producing reluctance torque T, which can be expressed with the following equation:
T=Pn(Ldxe2x88x92Lq)idiqxe2x80x83xe2x80x83(1),
where Pn is the number pairs of polarities, Ld and Lq are inductance of axis xe2x80x9cdxe2x80x9d and axis xe2x80x9cq,xe2x80x9d and id and iq are axis xe2x80x9cdxe2x80x9d current and axis xe2x80x9cqxe2x80x9d current. Equation (1) indicates that the difference between axis-d inductance and axis-q inductance, i.e., Ldxe2x88x92Lq, determines the performance of the motor. The flux barrier is prepared so that this difference Ldxe2x88x92Lq can be increased. This flux barrier provides a magnetic path across the slits and running along axis xe2x80x9cqxe2x80x9d with resistance, while securing a magnetic path running along axis xe2x80x9cdxe2x80x9d between the slits.
The conventional structure drives the motor only with the reluctance torque, so that the driving torque generated by the motor is obliged to be small. In fact, as some products (compressor, refrigerator, air-conditioner) driven by those motors need a larger driving torque, the reluctance motor is not always suitable.
Accordingly, there is a need for a motor that can produce a larger driving torque, using not only reluctance torque, but also the magnetic torque. The present invention addresses this need.
The present invention relates to an electric motor and a compressor with such an electric motor. Accordingly one aspect of the present invention is the motor, which as a stator and a rotor. The rotor has multiple rows of slits arranged along a radial direction thereof. A magnetic flux route is formed along the rotor between each adjacent slits.
A permanent magnet can be disposed at each of selected slits or at least some of the slits to generate magnetic torque. The selected slits can be disposed closer or closest to the center of the rotor. In this respect, the selected slits can have a void at each end thereof. In other words, the slits can be configured to drive the rotor with magnet torque in addition to reluctance.
The slits can have the following configurations. The width of the magnetic flux route can be widest adjacent to the selected slits. Moreover, the width of a bridge formed between an end of the selected slits and an outer rim of the rotor can be narrower than the width of other bridges formed between the respective slits and the outer rim of the rotor. The widths of the bridges formed between the ends of the slits and an outer rim of the rotor can widen progressively from the selected slits toward the slits disposed closest to the outer rim of the rotor. Specifically, the width of the bridge of the selected slits can be narrowest, and the width of the bridge of the slits closest to the outer rim of the rotor can be widest.
Moreover, the width of the magnetic-flux route adjacent to the selected slits can be widest at an end of the magnetic-flux route, adjacent to the outer rim of the rotor. Further, the width of the selected slits can narrowest at an end thereof, adjacent to the outer rim of the rotor. The width of the slits adjacent to the selected slits can be narrowest at an end thereof, adjacent to the outer rim of the rotor.
The winding of the stator can be formed by a concentrated winding. The permanent magnet can be one of a ferrite magnet and a resin magnet. Moreover, the rotor can be composed of a plurality of core sheets laminated together along a rotating axis of the rotor, with the slits being shifted from one another to provide a skewed alignment of the slits. The selected slits can be positioned closest to the center of the rotor, and the number of rows of slits can be between 3-5, 4 being optimal.
Another aspect of the present invention is a compressor that includes the electric motor described above.